1. Field of the Invention
The present invention relates to a process for the production of polyamides having a high viscosity in the molten state, which find application in the plastic products field, that is polyamides adapted to be transformed by the known technique of extrusion, molding, etc. into formed objects having high mechanical characteristics, which may be employed in various fields of industry such as for domestic, electrical, mechanical applications, etc.
2. The Prior Art
It is known that polyamides having improved properties of mechanical resistance may be obtained by the addition to the starting monomers of particular additives, such as phosphoric acid or alkylene carbonates, adapted to cross-link the polymer which is formed or at any rate to increase its degree of polymerization with respect to the conventional polymers prepared in the absence of such additives; polyamides having a higher relative viscosity and an improved resistance to mechanical stresses are thus obtained.
Such known processes however are not free from defects.
Thus for instance the use of phosphoric acid as an additive, gives rise to apparatus problems since said compound is considerably corrosive, and to technological problems since it is difficult to control the amount of said compound and excessive cross-linking and consequent lack of workability of the polymer may be caused thereby. When an alkylene carbonate is used as an additive, according to the known art it is introduced in amounts above 1%, directly in the polycondensation for the purpose of rendering the polymer, which is discharged at a relative viscosity of about 2.6, heat sensitive and therefore post-polycondensable in solid phase until a relative viscosity of 3.2 - 3.3 is reached. These processes however, further lead to drawbacks in the polymerization stage.
It was also known to improve the dyeability of the polyamides by adding ethylene carbonate to the monomers in amounts below 1%, but in that case no significant viscosity increase was obtained, and therefore this process is adapted only to cases in which such an increase is not desired.